core. files inside base directory

Hello, I have the core. Files inside the base directory of my postgres 10 setup . Would you please let me know the importance of these files . When they generate . Can we delete them ?

/base/

du -sh *

24M     base

8.1G    core.26166

1.5G    core.26258

8.1G    core.27849

12M     core.27951

4.3G    core.2951

8.1G    core.324

4.9G    core.3241

8.1G    core.3295

5.0G    core.3470

5.2G    core.3485

5.3G    core.3699

5.0G    core.3724

5.3G    core.375

5.0G    core.3935

5.3G    core.3960

1.2G    core.408

Thanks,