Yes, I will use it in signed applets. I don't know exactly what .rsa means, but according to wikipedia.org:
RSA is an algorithm for public-key cryptography. It was the first algorithm known to be suitable for signing as well as encryption, and one of the first great advances in public key cryptography. RSA is widely used in electronic commerce protocols, and is believed to be secure given sufficiently long keys and the use of up-to-date implementations.
RSA is the name of the standard public key encryption algorithm. I have never heard of a ".rsa" file. Ask whoever gave you the file what it really is.
In general, when you use RSA, you have key files (containing your RSA keys) and some cleartext file that you want to encipher. You use your keys, the cleartext and an implementation of RSA to encipher the file.
Or the other direction, you have a cipher text file, and your RSA keys, you use the RSA algorithm to decipher the file.
The keys are usually kept in a nasty binary format defined by RSA labs called PKCS #1 or #7, which is really ANS.1 BER/DER format. It is unreadible without serious binary mangling.
This is not really an advanced question, and probably belongs in Security or another forum