The word L' is used to replace ''Le'' and it is comparable to the word ''the'' in the English vocabulary. It is necessary to use them to make a complete correct sentence.
The apostrophe (') is a sign used to replace a vowel. So when the word following it starts with a vowel you need to use the short version of ''Le''
For example, Année starts with a vowel, so you need to use L'année. Le année makes no sense and it's grammatically incorect.
A correct sentence would be : L'année dernière a été amusante. (The last year was fun)
Here is a short list of word to help you understand the right way to use it.
Le chien ( the dog)
Le repas ( the meal)
Le chanteur ( the signer)
L'animal (the animal)
L'arche (the arch)
L'image (the image)
Notice the first letters after the L' or Le, it's the key to use that word correctly.