Called indefinite pronouns because they refer to things that are indefinite.
algo = something
nada = nothing
tudo = everything
cada = each
algo > Algo para beber? (Something to drink?)
nada > Nada faz sentido. (Nothing makes sense.)
tudo > Ele quer tudo! (He wants everything!)
cada > Ela sabe cada palavra. (She knows each word.)
You don’t need to modify these to agree with gender — they don’t change.