An associate is typically someone who does not have a significant time on the job or in the profession. In technical fields this title is usually 3-7 years to progress from junior though senior associate and into a base level engineer. Though that is not a hard and fast rule just how long it takes most people, I have seen, to be promoted.
A senior associate is someone who may be relatively new to the company and or field, but that has shown the ability to successfully complete their assigned work. Typically they would have stronger skills than an entry level associate but still have not shown the abilities that would merit a promotion to full software engineer. This does not mean that you do not have the skills just that they have not been demonstrated, but that you have been recognized as being at a higher level than a base associate. It also indicates a high level of expectations. A senior associate should not need much guidance in the completion of most tasks.