You are absolutely right. If the soil is rock hard then your lawn is most likely really suffering. There are a few different options:

1) A hollow tine aerator pulls out plugs of soil about a few inches deep and deposits them on the lawns surface. If you are reseeding or have quite a lot of lawn thatch then this is something good to do around once a year and will promote a thicker, healthier turf with better drainage. You can get manual (a bit like a fork) or mechanical/powered versions depending on the size of your lawn

2) A spike aerator which pushes a solid spike into the ground, usually a few inches. These can be used more often as the soil closes up again over time, especially when it gets moist. The spike pushes the soil sideways as it goes in and compacts the soil further but does allow air, moisture and nutrients in. Again, a manual version on shoes has been discussed and there are powered versions.

3) A blade aerator is similar to a spike but with a thin blade that causes slits in the turf that is effective immediately but closes up quite quickly. These are good for if you have a bowling green lawn and don't want holes or plugs of soil

In most cases, watering the lawn thoroughly the day or morning before will soften the soil up and make the process much easier!

Good luck.

More info about lawn aeration here.