Ukrainian, Russian and Belorus all originated from the Old East Slavic.
For this reason they are similar and have many similarities.
All languages evolve over time.
The point that is made is that Ukrainian and Russian are not identical which many people incorrectly assume.
There are many, many differences.
All Russian speakers in Ukraine will understand Ukrainian because it is the official language.
They just might not be good at speaking it because they dont used it regularly enough.
On the other hand, Russian's from Russian which have never been to Ukraine will struggle somewhat to understand Ukrainian, although they should get the general jist of the conversation.
The other point that must be considered is that Ukraine has had hundreds of years of unfair Rusification.
I was in Ukraine and was greatly disappointed to see 16 out of 19 tv channels in Russian language.
Many magazines are in Russian language.
After all those years of Rusification, Ukraine should begin a gradual process of Ukrainianisation which is happening (very slowly).
Russian media in Ukraine (tv, radio, magazine, movies etc) is a problem which must be changed to the Ukrainian language gradually over time.
Pencil in Ukrainian = Olivets
Pencil in Russian = Karandash
Pocket in Ukrainian - Kishenya
Pocket in Russian = Karman
Of course in Ukr = Zvychajno
Of course in Rus = Kanyechno
Hide in Ukr = Hovatysya
Hide in Rus = Pryatsya
The list of differences between Ukrainian and Russian are probably in the thousands.
I could write about 200 examples in the next 10 minutes.
But yes there are also many similarities which is undeniable.