There are four tones in Mandarin. The first flat one, high pitch, the second rising tone (like a question in English and Spanish), from lower to higher pitch, the third tone, falling rising, the forth one is the falling. It seems complicated when you read all this explanations, but it isn't when you try it. As to the Spanish students, due to the variety of Spanish pronunciation, it is much earlier to get the essence of the tones.
Does it matter if one cannot pronounce tones all correctly? Can they be understood? The answer is YES, they can be understood in a sentence! Of course when start learning it, we will try our best to pronounce each tone as correctly as possible because different tone represent different meanings. For example, mā 妈(the first tone) means mum; mǎ 马(the third tone) means horse. Then you might feel pressure before you open your mouth. But a sentence consists of phrases, called Ciyu, for example, ma1ma( mum), xiao3 ma3 (little horse). Usually a word isn't individual, it having its partners. Therefore you can make yourself understood well in a context even if you have a few pronunciations not 100% correct.
Another question that I have been frequently asked by students is the accent. As you know, people in different parts of China may speak Mandarin with slightly different accents, but not that big difference. It is quite common that language learners have some accents of their mother tongue when speaking Mandarin. The most important is to make a clear pronunciation. I have students from countries with different mother tongues, Russian, English, German, French, Spanish... I found an interesting phenomenon that the more complicated their first languages are, the easier to pronounce Mandarin. However, it is not absolute. To a large extent, it depends on efforts and practices one has made. Everyone can manage the tones for communication in the end.
It just a personal view on the tones, hope it helps you that is on the way or are going to pick up Mandarin. If you have any ideas and questions about Mandarin study, you can leave me a message here or my emial , we can share our opinions :)