In Gmail, you can add stars your emails. After adding stars to the emails, you can find them in the "Starred folder". By default, you only have "yellow star" to use in your Gmail. Please see below to add more stars.
Step 1: Login your Gmail account (please refer here to open one if you do not have an account yet);
Step 2: In the Gmail account, click the "Setting" button from the right-top corner;
Step 3: Click the "Settings" command from the drop-down list;
Step 4: Click the "General" tab in the settings menu;
Step 5: Scroll the cursor down to the "Stars" section, you have options of "1 star", "4 stars" or "all stars":
– When clicking "4 stars", the following 4 stars will be in use;
– When clicking "all stars", all stars will be in use;
Step 6: Click the "Save Changes" button at the bottom;
Step 7: Go back to Gmail "Inbox", and add stars to emails. If you have multiple stars, keep clicking the star icon until you see the one you want to use.