¿Le viste ayer?
English: Did you see him yesterday? (leísmo: 'le' instead of 'lo')
Leísmo, laísmo, and loísmo are special uses of the Spanish pronouns 'le', 'la', and 'lo'. They refer to how people sometimes use these pronouns differently from the standard rules, especially in Spain.
Normally, 'le' is for indirect objects (to/for someone), and 'lo/la' are for direct objects (someone or something). However, in some regions, people use 'le' for direct masculine persons (leísmo), 'la' for indirect feminine persons (laísmo), or 'lo' for indirect masculine persons (loísmo).
¿Le viste ayer?
English: Did you see him yesterday? (leísmo: 'le' instead of 'lo')
La di el regalo.
English: I gave her the gift. (laísmo: 'la' instead of 'le')
Lo dije a Juan.
English: I said it to Juan. (loísmo: 'lo' instead of 'le')
Lo vi en la calle.
English: I saw him/it in the street. (standard use)
Le envié una carta.
English: I sent him/her a letter. (standard use)