A tendency is an inclination to do something. For example, dogs have a tendency to bark at strangers and the mail man.
We all have tendencies: things we're inclined to do, or like to do, or just can't help doing. Morning people have a tendency to get up early. Basketball players usually have a tendency to go right or left. Optimists have a tendency to look on the bright side of things; pessimists have the opposite tendency. Other things have tendencies too, like how the stock market tends to go up and down.