Emphasizes a very small amount or degree. Often used to soften requests or statements.
Please wait just a little bit.
Please add just a tiny bit of sugar.
わずか is a more literary/formal word stressing scantiness, while ほんの少し is a common spoken emphatic phrase for 'just a little'.
たくさん means 'a lot', the direct opposite of ほんの少し's 'just a little'.
ほんの is a contraction of ほんに (truly), which itself comes from 本 (ほん, 'true') + に. Combined with 少し (a little), it literally means 'truly a little', emphasizing a very small amount.