Should is often used in conditional clauses expressing possibilities, suppositions etc. By using should in the if-clause we are suggesting that something is unlikely or not particularly probable.

If you should run into Mathews, tell him that he owes me $100. (= Your chances of meeting Mathews are rather slim, but if you MEET him, tell him that he owes me some money.)

More examples are given below

If she should come, ask her to wait. (She is unlikely to come, but if she COMES, ask her to wait.)
If they should attack us again, we will give them a warm reception. (They are unlikely to attack us again, but if they DO, we will make it memorable for them as well.)

Should can also go at the beginning of the sentence. In this case, if is dropped.

Should it rain, there will be no picnic today. (= If it should rain, there will be no picnic today.)
Should she arrive, ask her to wait.

Should she fail the test, she will lose her job. (= If she should fail the test, she will lose her job.)

Instead of should we can use happen in the if-clause.

If you happen to meet John, tell him that the meeting has been postponed.

OR

If you should meet John, tell him that the meeting has been postponed.

Should and happen to can be used together.

If you should happen to lose your job, what will you do?