What does "high" add to the meaning of this sentence of Tagore's poem?

Matt Ellen gives a nice answer to the question of what high means.

I would just like to comment on the interpretation of the whole sentence - in poetry, and especially free verse, the following is possible

Do not seat your love high upon a precipice - it is dangerous.

Do not seat your love upon a precipice just because it is high - and beautiful to look from above.

(other meanings of high can also apply - high in a sense of above, more worthy of others; putting something high and thinking highly of someone)

So, in the poem and especially a seemingly simple like the one quoted, you are often given the opportunity to contemplate not on a single possible interpretation, but on all of them and it can deliberately be written "ambiguously". Actually this kind of "ambiguity", in my eye, carries certain aesthetics, if all meanings convey different facets of the same idea or different, but consistent ideas.

The precipice is high, as in the same way a mountain is high.

So you should not put you love on the high precipice because that is dangerous.

I'm not very familiar with the poem, but this line could have a variety of meanings.

Because it is high

can be meant to make us visualize a risky, high precipice, or it can be meant to make us think of how something can be important, or both. For example:

• Do not put your love (the person you love, or your goal or dream) "on a pedestal". To place something or someone on a pedestal is to see no wrong in it, or to elevate it above all else. Perhaps Tagore means "don't put your love above you just because you can" - just because the precipice is high doesn't mean you have to constantly look up to it.
• Do not put your love (your goal or dream) too high, out of reach.
• Do not put your love (a person) in a dangerous place - perhaps meaning to look out for those you love.