Timeframe is one of the basic concepts of trading theory, and it is one of the most important ones to understand.
Timeframe describes the amount of time necessary to form the lowest element on the chart. For example, if you use candlesticks to visualize the price change, it would take 1 minute for the price to create one candle on the one minute (M1) time frame, and 1 hour for one hour (H1) timeframe respecitvely. The larger is the timeframe, more time it takes to create a candle. It works allmost the same way with other types of charts like bars or lines.
There is no one universal answer to the question of choosing the best timeframe. It would be better to say that it depends on your trading style and the strategy you use. For example, if you are active daytrader, your main timeframes would be M1 and H1, but if you are mostly interested in swing/mid-term trading, it would be better for you to use higher timeframes like H4 or D1. At the same time, despite your particular trading style, it is important to pay attention to various timeframes to be able to see the whole picture and to enter positions with an optimal risk-reward ratio to reduce risk and increase your profit potential.