When someone has Liked your Profile you will get a notification that lets you know that someone is interested in you. You can find these in your notifications tab (Icon shaped like a bell).

The notifications tab will show you the complete list of who has liked you, who wants to add you as a friend and anyone who has engaged on your post in the community.

To respond with yes, all you have to do is tap on the icon next to the notification. The app will give you a congratulatory message and then you'll have the opportunity to message.