Root tabs are granules of nutrients that contain macros and micros (dependents on the brand).
Plants like Vallsenria, Amazon Swords,
Crypts, Stem plants like rotala's and ludwigias, carpeting plants like
dwarf hairgrass and monte carlo are all plants that primarily take their nutrients through their roots in the substrate.
Because of this, we need to supply them with nutrients that the roots will suck up. Plants like Annubias and Java fern and bacopa typically take in nutrients from the water column through either their leaves or their roots (if the plant is does not have a planted rhizome) hence the usage of
liquid fertilizer.
Root tabs wise I usually use
Seachem Flourish Rootabs. API root tabs are also good root tabs but they contain more macro nutrients like nitrogen, and for the tank that I use rootabs in is overstocked so it doesn't call for an excess of nitrogen. Flourish Rootabs are rich in Micronutrients such as Iron and pottasium which is what my tank needs.