The phrase "full tsp" means a full teaspoon, which is a unit of measurement in cooking. It refers to the amount of an ingredient that fills a teaspoon completely without overflowing. Full definition
(I have a friend who put 1 full tsp of turmeric in water and drink it! (nutritionfacts.org)
If you're going to add the vanilla glaze, mix the three ingredients together in a bowl (add small amounts of the almond milk at a time, you may not need the full tsp) and wait until muffins are fully cooled to drizzle it on or else it'll just drip off. (pumpsandiron.com)
I used a full tsp of baking soda and added 1/2 tsp of xanthumn gum. (wholenewmom.com)