this is a diagram of 1st ionzation energies - the 5 x d orbs produce a really flat curve with very little energy difference between them. Hope this helps!!
eg A + B --> C + D
Because the have the same charge and they repel each other they need a lot of energy to collide and so this reaction has a high activation energy.
However if we used say Fe2+ as a catalyst then particle A can react with the Fe2+ in a redox reaction and form C + Fe3+. Then B reacts with the Fe3+ and reduces it back to Fe2+ and forms D.
The transition metal is acting as a go between giving an alternative path which has a lower activation energy. One of the species oxidises the transition metal ion and the other species reduces it back to what it was at the start so the transition metal ion is not permanently changed.