Target Loss Ratio (TLR) - The percentage of premiums allocated to pay claims. For example, if a group's TLR is 67%, the carrier is predicting the group will claim $0.67 for every $1.00 of premiums paid. For most carriers there is a direct relationship between the size of the group and the TLR assigned to that group.
A common question here is "what happens to the other 33%?"
The amount beyond the TLR is used to cover the carrier's service and administration, cost of risk, IBNR's, profit, and broker commissions. 

Incurred But Not Reported (IBNR) - When a policy is terminated the carrier is still responsible for paying claims incurred prior to the termination date, so this is a float that insurance carriers are required to build into a group's premiums to cover claims that plan members incur but delay sending in.

Trend Factors - inflationary pressures related to the (usually increasing) costs of medical services as well as predicted increases in the use of these services by plan members. For example, if the cost of drugs and the use of drugs is increasing such that drug claims on a plan can be expected to increase by 20%, the carrier will use 20% as a trend factor and apply this towards the drug rates. Healthcare usually trends annually at 5 - 15%, Drugs at 15 - 25%, Dental at 4 - 10%.

Pooling - the concept of spreading claims (and risk) over a large number of insured persons. A benefit that is "fully pooled" will not increase or decrease in premium rate when claimed against. When a benefit is "fully pooled" the rate will not change for only one group when someone makes a claim. Life, AD&D, LTD, CI, and sometimes STD are pooled benefits. Some carriers will pool all benefits for groups of certain sizes.

Experience Rating - the concept of applying claims experience to calculating rates for a particular benefit.

Manual Rates - manual, or "book" rates, are the rates calculated by an insurance company based on its prediction of benefit use without regard for a particular group's real claims experience. Manual rates would be used on a group with no previous benefits plan and in part at each renewal for smaller groups, where their claims experience cannot be statistically relied upon.

Experience-based Rates - the opposite of manual rates, these are the rates determined by the carrier using only the group's real claims experience. For smaller groups, this is usually combined with the carrier's manual rates because the claims experience from smaller groups may be statistically inaccurate. A larger group, however, is more likely to follow a pattern of claims from year to year, so its rates are weighted more heavily on the group's experience.

Credibility - the ratio (expressed as a percentage) of how much a group's claims experience should affect its premiums for a particular benefit. Assigned by the carrier to each group based on the size of the group and how many years the benefit has been in force, a group's credibility is used in determining new rates at each renewal. Credibility varies from group to group, and sometimes from benefit to benefit. For example: If a group's credibility for healthcare is 70% at renewal, the new rates will be based 70% on the claims experience and 30% on the carrier's manual rates.