The core of all algebra concepts is solving basic algebraic equations and expressions, which are defined by the inclusion of variables (e.g. x and y) and numbers.
A single-step equation may look like: 2+b=c
To solve for b, you must subtract 2 from both sides. In all of algebra, what you do to one side you must do to the other.
You get: b=c-2
Multi-step equations require, as the name suggests, more than one step to solve.
Here's an example, solving for a: 2a+7=b
First you subtract 7 from both sides: 2a=b-7
Then divide both sides by 2: a=(b=7)/a