It is important to know what the status of a patent (application) is. A patent may have already expired, in which case it will no longer cause you any problems. It is also possible that a filed patent application has not yet been granted. By consulting the correspondence between the patent applicant and the patenting body, you can get an idea of the likelihood of a successful grant of patent. It is also possible to predict how far the protection of the patent will extend, which is often not as far as the original patent application might imply.
Expired patents: As soon as a patent has expired, the protected technology can be freely used.