An EQ pedal is simply put a pedal that allows you to craft the frequency range of your sound by cutting or boosting specific frequencies as determined by the pedal designer. They can be very simple i.e. focusing on one frequency category (say Mids only) or they can be more versatile and operate over 3 frequency bands Bass, Mids and Treble. If you need more control, some EQ pedals offer 10 or 12 bands of fixed frequency shaping. Some even go one step further and offer parametric equalisation where you have full control over the frequencies chosen, the gain of those frequencies and the bandwidth of the frequencies adjusted.
If your amp has EQ controls and so do your pedals, why do you need a standalone EQ pedal? A standalone EQ pedal gives you control over your sound in a way that a lot of pedals or amps just don’t. The ability to cut OR boost the frequencies to taste as opposed to just cutting them makes the EQ more powerful than the other tone shaping elements of your rig (say the amp or the pedals you have). If your guitar sound is booming in one venue, you can dial down the Bass, if you aren’t cutting through the mix you can accentuate the mids. An EQ pedal allows you to fine hone what you want to sound like and more control in your hands has to be better.
Some drive pedals have very specific EQ characteristics, for example the legendary Tubescreamer and the ubiquitous Klon Centaur are actually often used as a single band EQ to accentuate the Mids for a guitarist. Often these pedals are used with mid scooped amps to fill the frequency deficit common with that amps design, or they are used to lift the mids with the aim of giving space in the mix for a solo to shine through. A well-designed EQ can do all of this and more so it is my personal belief that if you like your core driven sounds, then an EQ pedal is better suited to provide the tonal sculpting you need.
If this is your first foray into EQ pedals, give the ThorpyFx Team Medic a go... 3 band ACTIVE EQ, boost, buffer and more boost. its a real Swiss army knife of a pedal.
Hopefully that swift run through will allow you to get the EQ you need, and if I could recommend one other thing, stick the EQ at the end of your pedalboard, it can then act as a post EQ for your Pedals and a Pre EQ for your amp.