Backbeat Groove, 2 & 4 Hit Strums
Adding a percussive hit to your rhythm really adds a great groove to your playing. Most people find it a little tricky to get it together but it's just slow and careful practice to get the mechanics sorted out and then once you get it right, you just have to keep doing it.
The "Backbeat" is accenting beats 2 and 4 - the same beats that a drummer will play the snare drum on. You'll often play more bass strings on beats 1 and 3 which also copy the drummer who would usually play the bass drum (big one played with their foot) so adding the accent on beats 2 and 4 brings a kind of drumming element to your groove and will get people dancing!
To make the accent you can play the notes harder, or mute the strum so you get a 'hit', which is my preferred method most times - sounds more drum-like. There are lots of ways of doing it, but the video should explain the mechanics easier than I can in text!
Once you got the basics together, work on locking into the groove and making it feel good :) it's something I go on about a bit, but it makes a big difference!
Here's a diagram showing the accent with the X.