What does SCM mean in Jenkins

SCM = Source Control Management

From jenkin's tutorial

Section: Create your Pipeline project in Jenkins

Step 6: From the Definition field, choose the Pipeline script from SCM option. This option instructs Jenkins to obtain your Pipeline from Source Control Management (SCM), which will be your locally cloned Git repository.

SCM in Software engineering stands for Software Configuration Management, see here for more details.

It also stands for Source Control Management in some Continuous Integration tools like Jenkins, please see the accepted answer.