what does ‘not in tenants in common but in joint tenants’ mean in texas

It means it is held  (or owned) as joint tenants, not as tenants in common. Tenants in common have essentially their own, divisible interests in property: if there are two tenants in common, each is a half-owner and can sell, give away, will to someone after their death, etc. their 50% interest, and the other owner has no say over what they do with it. 
But joint tenants have indivisible interests: when one joint tenant passes away, the other one instantly and automatically, without probate or the property interest going through the deceased's estate, becomes the sole, 100% owner. That means one of the joint tenants cannot will or leave his/her interest to anyone else; it also means, because the likelihood of the other owner getting the interest if/when the first one dies is affected by who that first owner is (e.g. his/her age, or if there is one other joint owner or multiple ones), he or she cannot even sell or gift his/her interest, since changing the identity or number of joint tenants affects the likelihood of a joint tenant becoming sole owner on the other(s) death. So both joint tenants have to sell, gift, etc. the property together; neither can sell, gift, will, etc. their interest separately.