Definition of early
When a player plays "early", it means that he is in an early position, in other words, he is among the first to play preflop. The earliest position is the Under The Gun (UTG). He is first to speak preflop. Depending on the number of players at the table, there may be a UTG+1 and UTG+2. These three positions are usually called "early" positions. Then there are the "middle position" players (MP1 and MP2) or "hijack" and "lowjack" depending on the vocabulary you use. So, to recap, early positions, then middle, cutoff, button and blinds.
What is the implication of playing early?
When playing early (EP), we have to consider several aspects. First, we speak first without knowing what the other players have done. We have no clue about the strength of their hand. We have to be careful with these positions. While a hand like J7 suited may very well open from the button, such a hand in UTG position or in early position in general will be almost impossible to make money on. In early position, you will want to be more conservative, especially if you are a beginner. If you can play more than 50% of your hands profitably from the button, it is not a mistake to play as little as 12% of your UTG hands. On the other hand, when you bluff, if you are early, you will be given a lot of credit.