Gesso is a primer which you can apply to almost any surface and then paint over with acrylic paints. It gives the surface a "tooth" for the acrylic paint to adhere to. So, if you wanted to turn that old vinyl record into a piece of art, just gesso it first.
You can apply it with brushes, palette knives or even an old credit card. You're only looking for a thin layer - you're not wanting to pile it on. Its, as Dina Wakley says, is the underwear for your page. Where you start, and dress everything else on top.
When using it on book pages, materials such as canvas, or thin paper pages it makes them stiffer, and so easier to work on and prevents the fibres of the surface just sucking up your paints, allowing them to move around more and, importantly, meaning you need to use less paint as a result.
Gesso is available in white, black, neutral gray and clear, but you could mix a little amount of paint in with your gesso (clear or white) to get a different colour tint.
If you are lucky enough to get your hands on an old ledger and want to turn it into an art journal without losing too much of the journal itself, prime it with clear gesso. That way you will keep all that old book goodness in your work.
If you're applying gesso to fabric, then use a brush, making sure you wash it out properly afterwards. If, however you are applying it to a page I would recommend using an old credit or gift card to spread the gesso. Pour a small amount onto your page and use the card to spread it out. You'll get a nice, quick drying, thin layer.
You can also use your gesso to create texture on your pages. Simply pour a little more than normal on to your page, then using either an old credit/gift card, or even better a palette knife, spread the gesso out as if you're spreading butter on to bread, then leave it to dry.
While it's still wet you can stamp into it. Try using lids or rubber stamps. The textures will really leap out at you once you apply a wash of acrylic paints over it. You can use rubber stamps to create texture - but you MUST wash them straight away if you ever want to use them with regular ink pads again.
We stock gesso, along with other mediums, by Golden, Liquitex, Dina Wakley, Ranger and Claudine Hellmuth, with all prices to suits your budget.