Learn Hardcore Norwegian: Vi må ta hensyn til tiden, ellers rekker vi ikke toget. - We must take the time into consideration, otherwise we won’t catch the train.

Ta hensyn til is a fixed expression meaning to take into account / to consider.
Word-for-word it’s like take consideration to, which doesn’t map neatly to natural English, so it’s best learned as a chunk:

å ta hensyn til + noun = to take account of + noun
Examples:
Ta hensyn til været. = Consider the weather.
Hun tar hensyn til barna. = She takes the children into account / is considerate of the children.

(You’ll also see the noun hensyn used in other set phrases, but ta hensyn til is the most common.)

Why is it hensyn and not et hensyn / hensynet?

In ta hensyn til, hensyn is typically used as an uncountable, idiomatic noun (more like “consideration” in general), so it often appears without an article.
You can use the countable form when you mean “a consideration/a factor”:

et hensyn = a consideration (one specific factor)
flere hensyn = several considerations

But in the idiom ta hensyn til, the bare form hensyn is the standard choice.

What is the grammar of må here—does it mean “must” or “have to”?