Conditional statements allow you to decide whether or not to execute a set of commands based on a series of factors.
For example, if you need to filter out basketball players based on number of rebounds in a game, you can use if/else/elseif statements.
If you want to execute a set of commands only if a player had 10 or more rebounds in a game, you can enter the following:

if ($rebounds > 9) {
echo "This player had 10 or more rebounds";
} else {
echo "This player had less than 10 rebounds";

If we pass this conditional statement a player that had 7 rebounds, we get the following result:

This player had less than 10 rebounds

You can also add elseif statements to include more than two possible options. Just replace else with elseif and add another condition.
Make sure the last statement is an else statement though, otherwise it will keep looking for conditions.
If we add elseif ($rebounds > 4) to our previous conditional statement, we know get:

This player had 5 or more rebounds, but less than 10

You can use a case statement to replace an if/elseif/else statement. It can save some typing, but needs to go through each case you want covered.
For example, you start with switch ($rebounds) {, and then run through each possible option and want commands you want executed one by one.
You can use a default case at the end similar to an else statement, where anything not covered by the cases will use the default case commands

This player had 7 rebounds