In plain and simple terms can someone help me to understand the mechanism of hydroboration oxidation? Thanks x

I've done this in full for alkynes here... no simpler terms than a diagram!
http://socratic.org/questions/when-does-hydroboration-oxidation-in-alkyne-occur

1. #"BH"_3# complexed with #"THF"# (Tetrahydrofuran), generally written as #"BH"_3cdot"THF"#.

2. #"NaOH"(aq)#, #"H"_2"O"_2#

Notice that the aqueous label is necessary because water does participate in this reaction at the end. (If you don't want to put it, then you need to additionally specify #"H"_2"O"# as a reactant.)

The difference is that for an alkene mechanism, you start with an alkene, you have one less single bond in each step, and you end with an alcohol, rather than an enol.

#"BH"_3# adds onto the alkene via a syn addition (#"BH"_2# and #"H"# on the same side), and ultimately the water adds onto the alkene anti-Markovnikov (addition of #"OH"# onto the LESS-substituted carbon).