「たいてい」 means 'usually' or 'mostly'. You use it to describe something that happens often or is generally true. It suggests a high frequency, but not 100% of the time. For example, you might use it for your daily habits.

ほとんど(ほとんど)の 場合(ばあい)に、いつもではないけれどよく 起こる(おこる)ことを 表します(あらわします)。