It means if the key exists in the hashtable then it returns true otherwise
false.

If this helps, I've pasted the underline code which was produced by
Reflector.

public virtual bool ContainsKey(object key)
{
     uint num1;
     uint num2;
     Hashtable.bucket bucket1;
     if (key == null)
     {
           throw new ArgumentNullException("key",
Environment.GetResourceString("ArgumentNull_Key"));
     }
     Hashtable.bucket[] bucketArray1 = this.buckets;
     uint num3 = this.InitHash(key, bucketArray1.Length, out num1, out
num2);
     int num4 = 0;
     int num5 = (int) (num1 % bucketArray1.Length);
     do
     {
           bucket1 = bucketArray1[num5];
           if (bucket1.key == null)
           {
                 return false;
           }
           if (((bucket1.hash_coll & 0x7fffffff) == num3) &&
this.KeyEquals(bucket1.key, key))
           {
                 return true;
           }
           num5 = (int) ((num5 + num2) % ((ulong) bucketArray1.Length));
     }
     while ((bucket1.hash_coll < 0) && (++num4 < bucketArray1.Length));
     return false;

not at all.
it means the query algorythm has a speed independant of the number of
element i the hashtable.

O(1) means the lokup time doesn't depends on the number of element
O(n) means it varies linearly with the number of element
O(n2) means it varies as the square value of the number of element
best sorting algorythm vaires in
O(n ln(n))