This structure can be used to make a verb the topic of the conversation, so you can describe it in more detail.
Tennis is fun.
Playing tennis is fun.
Watching tennis is fun.
The two later examples are more specific in that that they say what the speaker likes to do, rather than just what thing he/she likes.
other adjectives that are often used to describe verbs include むずかしい, たのしい, きけん, たいへん.