You are probably quite familiar with reading and playing two notes together in drum kit music. In fact, in grooves this is something that happens the majority of the time, you'll have a constant right hand part then written underneath those notes are bass drums and snares drums. In these cases the two notes are split in to different 'voices' so have there own note heads and stems. It's also not uncommon for you to see two notes written in the same voice, such as a bar of snares and floor toms played as eighth notes. In this case each note will have its own note head, but they will share a stem.
Something you will sometimes come across is two note heads that share a stem. These are called 'Unison Notes' and you will see them most often in grooves where you Ride On The Snare. All you do here is play both notes at the same time with two different hands, like you would with a Flat Flam.
For example, in the image below you have two solid note heads written on the snare that both share a stem. So to play this you will hit both hands down on the snare at exactly the same time.
Below I have written a few more examples of unison notes.
Quaver Unison Note On The High Tom
Semi Quaver Unison Note On The Floor Tom
Minim Unison Note On The Hi Hat
Semi Breve Unison Note On The Mid Tom
This idea isn't just related to drum kit, it applies to all instruments. The rule here is that for notes to be 'unison' they have to be the exact same note played at the exact same time.