A candlestick shows four prices for a period of time: where the price opened, the highest point it reached, the lowest, and where it closed. The body of the candle spans the open and close. The thin lines above and below, the wicks, show the high and low. A candle that closed above where it opened is usually drawn hollow or green; one that closed lower is filled or red.
A one-hour chart shows one candle for each hour. A daily chart shows one for each day. Every candle on a chart summarises the price movement of its period, and the period is the timeframe.
The standard timeframes
MetaTrader 5 offers timeframes from one minute to one month. The common ones are M1, M5, M15 and M30 for minutes, H1 and H4 for hours, D1 for daily, W1 for weekly and MN1 for monthly, with H2, H3, H6, H8 and H12 also available.
The trade-off between them is simple. Lower timeframes give more candles and more signals, and each signal contains less information and more noise. Higher timeframes give fewer, slower signals, each of which represents more trading and is less affected by the spread, the rollover and the odd spike. A strategy on the daily chart might trade a few times a month. A strategy on the one-minute chart might trade a few times an hour.
When a candle closes
Most trading decisions are made on a closed candle, because until a candle closes its close, high and low can still change. The moment a candle closes is when a signal is confirmed and when most rule-based systems act, at the open of the next candle.
For candles up to H1 that moment is the same at every broker. For H4, daily and weekly candles it is not, because the candle boundaries depend on the broker's server clock.
Server time and the daily candle
A daily candle runs from midnight to midnight in the broker's server time. Brokers set their server time differently. Many use a time zone chosen so that the daily candle closes at 5pm New York, the end of the forex trading day, which produces exactly five daily candles a week. Others use a different zone and produce a short sixth candle for the Sunday evening session. The two brokers will show different daily highs, lows, opens and closes for the same day, and the four-hour candles will start at different hours.
For a trader glancing at a chart the difference is small. For a strategy whose rules depend on the daily close or the daily high and low, it can change which trades are taken.
A strategy buys when today's close is the lowest close of the last ten days. At a broker whose daily candle closes at 10pm UK time, today's close is the price at 10pm. At a broker whose daily candle closes at midnight UK time, it is the price two hours later, after a period of thin trading. On some days the two brokers will see different ten-day lows and different signals. Over a year the strategy will have taken a somewhat different set of trades at each, and its results will differ.
An EA built on daily or weekly candles is built on a particular candle alignment, and its results are only reproducible at brokers with the same alignment. The vendor's documentation should say what server time the EA was developed and tested on, and whether the EA handles a broker whose time differs. Some EAs compute their own candle boundaries from a fixed reference time so that the broker's clock does not matter. That is the more careful design.
The broader point is that the timeframe an EA trades on says a lot about it. Systems on daily and weekly charts trade rarely, hold for days, and are largely indifferent to spread and slippage. Systems on one- and five-minute charts trade constantly and depend on execution quality that varies broker by broker. Both can be legitimate. The second kind is far harder to run profitably outside a backtest.