As you have already been informed, there is a dictionary and half of it is actually a description of the grammar. As such it is an essential tool for helping to learn Klingon, no matter where or how you are learning it.
When the dictionary was written it was sort of written as a joke. The linguist writing it (Dr. Marc Okrand – a friendly, intelligent, and funny guy if you ever get the chance to meet him) took the language seriously (mostly), but assumed that no one would actually go so far as to learn this language that he created, so did not always take the instructional role of the book so seriously. Unfortunately, this means that it is not always so good at actually teaching you how to speak the language. All the information is there, but it takes some effort and dedication to use it to learn to speak the language. There are other tools out there. The best may be the Learn Klingon Online course, right here on the KLI webpages. But you will still probably need The Klingon Dictionary as an additional reference to look up the details of something you don’t understand or need to refresh your memory of.
When you ask whether it’s easy to understand, do you mean listening or reading?
It’s fairly easy to read. The writing system is based on Latin script (the “English alphabet”), entirely phonetic (you pronounce how you write and write how you pronounce), and regular. There’s even software to parse sentences for you.
Listening is another matter. It takes lots of practice to understand spoken Klingon.
Yes, there’s a dictionary:
You can buy it and learn Klingon using it. How easy it is to understand depends on you. The greater your background in linguistics and/or learning languages, the more familiar you are with agglutinative languages, etc., the easier learning Klingon will be. Some people will find Klingon to be a relatively easy language to learn. It’s completely regular and has a complete if minimalist grammar. Some people will find Klingon very hard. The word order is more-or-less backwards when compared to English and most European languages, and the language does the bulk of its grammatical work through prefixes and suffixes.
Either way, if you get stuck, there are resources available to help, not least of which is this very section of the KLI website. The folks here are very friendly and quick to answer questions.