Axis? I believe you mean the Central Powers.
The first attack the British made caught both the Germans and the British off guard because it worked! The Germans realizing that their rifles and machine guns were not working against them retreated. The British half thinking the tanks wouldn't work didn't bother to have units ready to follow the tanks and exploit the breech in the line. In a few days the Germans had pushed them back.
Were tanks frightening? Well, if you are an infantry man with just a rifle a tank is a pretty scary thing to have come at you.
As the war went neither side at first was quite sure how to use tanks. Often there would be a great deal of shelling before sending out the tanks. Being Western Europe there was often a lot of rain at the same time, the tanks would bog down in the mud and artillery would take them out. took awhile for the allies to learn to concentrate the tanks and often just sent them out one or two at a time to support the infantry and artillery would take them out first.
Some of the tanks were very badly made. Some wouldn't even stop rifle bullets. Bullet come in one side and bounce around inside. For some odd reason some US Tanks had their engine exhaust port inside the tank itself. So, the tank crews got carbon dioxide poisoning, some even died in the tanks. The crew would often be in hospital for a week after the attack.
A few British generals finally figured it out. They planned an attack with concentrated tanks and had a new lighter, faster moving tank called a whip it.
Their idea would be to attack at first with minimal artillery at first, let the whip its break the barb wire and get behind the lines.
The larger slower tanks would escort the infantry in and aircraft would try to stop German reinforcements.
I think they got one successful attack off and before a second larger one could take place the war ended.
^ these answers I've given before should answer your question! /u/NMW has also given answers on Tanks in WWI, that are worth looking at.
Hope that helps!