If you continue your browsing, specifically here on HoFo, you'll see that this offering is part of a broader deal from America Movil, the company that TracFone, Net10, and Straight Talk are subsidiaries of. Here on Howard Forums the offer/deal you mention is commonly referred to as the "BYOD (for bring your own device) - SIM" deal. It is offered on both Net10 ($50 monthly) and Straight Talk ($45 monthly), with a choice of either AT&T or T-Mobile service (determined by the choice of SIM). You should find out which of these carriers provides the best service in the area you intend to use your phone (live in and/or work/travel in), and choose the SIM accordingly. DO NOT select the "Unlocked" option, as you will then be giving the company discretion in choosing who IT thinks will be the best for you based on the zip code you input on the website, which is not always the way you would want it to go.
Also, for the carrier (the underlying one -- AT&T or T-Mobile) that you determine is best for your needs, you should pair up the SIM with a phone that is completely compatible with that carrier's 3G/4G radio frequencies, meaning use an AT&T SIM in an AT&T phone, or a T-Mobile SIM in a T-Mobile phone, OR use a phone (and yours might be one of them -- I don't know) that is SIM unlocked and is capable of use on either carriers' 3G/4G service.
Hope that all helps, and wasn't too much!