A function $f$ with rule $y=f\left(x\right)$ is called
increasing if the value of the function becomes larger as $x$ increases;
decreasing if the value of the function becomes smaller as $x$ increases.
A function can have
a maximum if there is a transition from increasing to decreasing without a break in the line;
a minimum if there is a transition from decreasing to increasing without a break in the line.
The points at which these transitions occur are called the extrema (singular: extremum) of the function.
You can see different types of increase and decrease in a function:
a constant increase (decrease) if the function continues to increase (decrease) with the same (constant) rate;
an accelerated increase (decrease) if the rate of increase (decrease) becomes larger as $x$ increases;
a decelerated increase (decrease) if the rate of increase (decrease) becomes smaller as $x$ increases