我很想去登一下长城。I very much want to climb the Great Wall (once).

你愿意再去一次北京吗？Are you willing to go to Beijing again?

这部电影我完整地看了两遍。I have watched this film from beginning to end twice.

下, 次, 遍 in the above sentences are used for counting the number of times an action occurs: they are measure words for actions. Verbal classifiers always follow the verb. Other verbal classifiers include: 回, 趟, 番, 声, 顿, 阵, etc. They all mean 'time(s) ' but have slightly different connotations.

次 and 下 can be used with most of the verbs. 一下 can also denote a brief duration apart from 'once'.

番 indicates that the action is tiring or time-consuming.

回 implies that a return journey is involved.

趟 can only follow certain verbs of movement like 走 and 跑.

遍 emphasises the concept of 'from beginning to end', 'completely'.

e.g. 'I explained it to him' can be translated in different ways.

我给他解释了一番。 (a lot of things were explained and it took quite some time)

我给他解释了一下。 (only certain things were explained and it was quick)

我给他解释了一次。 (just 'once', the content is not concerned)

我给他解释了一遍。 (thing were explained from beginning to end)

More examples of verbal classifiers used in sentences:

这个孩子被他妈妈骂了一顿。This child was told off by his mum.

你把我吓了一跳！You startled me!

她看了他几眼。She threw several glances at him.

他猛踢一脚，球进了！He kicked the ball hard, and scored a goal!

Note that certain verbs and nouns can be borrowed as verbal classifiers, as shown in the above examples.
跳 to jump, 眼 eye, 脚 foot.