It may be used free of charge, even by commercial users.
Full source for the operating system is freely available, and the minimum possible restrictions have been placed upon its use, distribution and incorporation into other work (commercial or non-commercial).
Anyone who has an improvement or bug fix is free to submit their code and have it added to the source tree (subject to one or two obvious provisions).
It is worth pointing out that the word ``free'' is being used in two ways here, one meaning ``at no cost'', the other meaning ``you can do whatever you like''. Apart from one or two things you cannot do with the FreeBSD code, for example pretending you wrote it, you really can do whatever you like with it.