To "steadily increase" means to keep increasing by the same amount, again and again. Here's an example of some steadily increasing numbers:

20 > 25 > 30 > 36 > 41 > 45

Here are some numbers that are increasing, but not steadily:

20 > 29 > 27 > 28 > 40 > 40

Numbers can also steadily decrease or fall, which means to go down little by little.