An equation may be true or false.

For example,

1+1=2

is true but,

1+1=3

is false.

Sometimes an equation may be invalid, for instance:

2 apples = 3 oranges

Because apples and oranges are different things and cant be compared (although we could compare the price or the weight of apples and oranges).

Normaly we will be assuming some equations are true and then apply a set of rules to find the value of an unknown or prove a relationship.