Low Application Memory
At times, your computer may give you an alert about low application memory, or your computer may just be running slowly; in this case, you may have too many applications running, or some of your applications may be taking up excessive memory. If feasible, try to close some applications. In some cases, it might be enough to simply close the problematic applications and reopen them, or close some tabs in a web browser. In more extreme cases, you may want to reboot your computer entirely.
This kind of issue can also show up if your computer has been running for a long time (i.e. multiple days) without being shut down. For this reason, it is recommended that you shut your computer down at the end of each day, if possible. Even simply rebooting your computer can help.