What is the actual meaning of being a research assistant? I have only heard of masters in science and PhD.
A graduate research assistant—regardless of field–is someone who is carrying out research, normally as part of studies that will ultimately lead to either a master's or a PhD (or degree of some other kind). In general, a "graduate research assistant" (or other similar terms) are administrative in origin—it's the name for the "job" that graduate students have while they progress toward their degrees.
Adding to what @aeismail wrote: all PhD students have to do original research in order to graduate. Many get free tuition plus a stipend in exchange for carrying out 20 hours per week of assorted teaching duties, such as grading homework for a professor. They are called "teaching assistants." However, some of them get the free tuition and the stipend, but with no teaching-related work duties, and they are called "research assistants." Some research assistantships do not involve any work duties at all. Some do, but not more than 20 hour per week. Some examples of research assistant duties: help less experienced students in the group; keep the lab well organized and tidy; perform certain computations as requested by the professor; create and/or maintain a website for the professor; assist with a research project (other than the student's own topic).