Open Access is a set of principles and practices that aim to remove any financial, technical or legal barriers to accessing any published scholarly outputs. Open Access publications are available "Gratis" (without charge), and "Libre" (without restriction on re-use).
If a publication is defined as Open Access, then anybody, anywhere in the world, with an Internet connection can download and reuse this publication without any legal or financial impediment.
There are two main routes to Open Access. Gold Open Access refers to the practice of paying Article Processing Charges (APCs) at the point of submission. The intent is to pay for the associated costs of publishing up front, removing any need to charge at the point of consumption. More information on your Gold Open Access options are available here.
Green Open Access, on the other hand, does not entail paying APCs but instead publishing in the traditional manner (in a subscription charging venue) and then uploading (typically) your final working copy of the output to an Institutional or Subject Repository, or personal website. Whilst no charge applies to Green OA, it is usually only the author's accepted manuscript that can be uploaded, and in most cases an embargo (temporary restriction of access) will apply. More information on your Green Open Access options are available here.
Removing the paywall barrier from your research benefits your colleagues and peers all over the world, particularly in territories where research budgets and lack of collective bargaining mean severely restricted purchasing power. Funders are increasingly concerned about value for money and the negative effect that paywalls place on 'bang for buck' from grants. More information on funder's OA policies is available here. Besides the purely altruistic nature of removing the paywall there is, of course, payback for the author in the form on increased citations accruing from increased availability. More information on the OA citation advantage is available here.