Thevenin circuit, basic laws are confusing me
considering V(ab), so +5V means (a) is 5V above (b) 


Yes.


  and -5V means (b) is 5V below (a) ?


No, that would be the same as (a) being 5 V above (b). It means b is 5 V above a. 

By convention \$V_{ab}\$ means \$V_a - V_b\$, so if \$V_{ab} &gt; 0\$, it means \$V_a &gt; V_b\$ and if  \$V_{ab} &lt; 0\$, it means \$V_b &gt; V_a\$.